BLKPG changes pushed

[Jim Meyering]

I'm not happy with these changes yet, mainly because of the lack of tests,
but also because I just tried to add a couple partitions (in free space),
and each time, I got this:

    $ sudo ./parted -s $dev u B mkpart F13-root $s2 $e2
    Warning: WARNING: the kernel failed to re-read the partition table on
    /dev/sda (Device or resource busy).  As a result, it may not reflect all
    of your changes until after reboot.

I thought the intent was to avoid such warnings when they do not apply.
The kernel was indeed notified of the creation of the two new
partitions, and I was able to create file systems on them.

Regardless of all that, I've just pushed that series from "next",
after rebasing against master.

We'll work from there.